Field of the Invention
The invention relates to a sensor device for detecting the direction of an external magnetic field through the use of at least one sensor element, including a multilayer system having a very great magnetoresistive effect (GMR) and having at least one soft magnetic measurement layer, at least one relatively harder bias layer with a predetermined direction of magnetization and at least one nonmagnetic intermediate layer disposed therebetween. A corresponding sensor device can be inferred from International Publication No. WO 94/17426, corresponding to U.S. Pat. No. 5,650,721.
In layers of ferromagnetic transition metals such as Ni, Fe or Co and alloys thereof, there may be a dependence of the electrical resistance on the magnitude and the direction of a magnetic field permeating the material. The effect occurring in such layers is called the "anisotropic magnetoresistance (AMR)" or "anisotropic magnetoresistive effect". In physical terms, it is based on the differing scatter cross sections of electrons with differing spin and the spin polarity of the D band. The electrons are designated as majority or minority electrons. In the case of corresponding magnetoresistive sensors, in general, a thin layer of such a magnetoresistive material having a magnetization in the layer plane is provided. The change in resistance upon rotation of the magnetization with respect to the direction of a current inducted through the sensor may then amount to a few percent of the normal isotropic (=Ohmic) resistance.
Furthermore, magnetoresistive multilayer systems are known which include a plurality of ferromagnetic layers that are disposed in a stack and which are separated from one another in each instance by metallic, nonmagnetic intermediate layers and the magnetizations of which in each instance preferably lie in the layer plane. In that case, the thicknesses of the individual layers are markedly smaller than the mean free path of the conduction electrons. In such multilayer systems, in addition to the mentioned anisotropic magnetoresistive effect AMR, it is now possible for a so-called "giant magnetoresistive effect" or "giant magnetoresistance (GMR)" to occur (see, for example, Published European Patent Application 0 483 373 A1). Such a GMR effect is based on a scattering of majority and minority conduction electrons "of different strengths" at interfaces between the ferromagnetic layers and the intermediate layers adjacent thereto as well as on scattering effects within those layers. In that case, the GMR effect is an isotropic effect. It may be considerably greater than the anisotropic effect AMR. In general, reference is made to a GMR effect (at room temperature), if it adopts values which are markedly above those of AMR single layer elements.
In a first type of corresponding multilayer systems showing a GMR effect, in the absence of an external magnetic field, adjacent magnetic layers are oriented to be magnetically antiparallel by reason of a mutual coupling. That orientation can be converted, by an external magnetic field, into a parallel orientation. In contrast, a second type of GMR multilayer systems has a so-called bias layer, which is magnetically harder than an existing measurement layer which is magnetically as soft as possible. In that case, the measurement layer and/or the bias layer may also be replaced in each instance by a plurality of layers stacked to form a packet. However, in the text which follows only individual layers will be assumed in each instance.
In the case of such a layer system of the second type, the measurement layer and the bias layer are mutually magnetically decoupled by a nonmagnetic intermediate layer. In the absence of an external magnetic field, the magnetizations of the two magnetic layers then have some relationship to one another, for example antiparallel. Under the influence of an external magnetic field H.sub.m (which is the component of the measurement field in the layer plane of the measurement layer), the magnetization M.sub.m of the soft magnetic measurement layer then becomes oriented in a manner corresponding to the direction of the magnetic field, while the orientation of the magnetically harder bias layer remains virtually unchanged. In that case, the angle .phi. between the magnetization directions of the two layers determines the resistance of the multilayer system: In the case of a parallel orientation, the resistance is low, and in the case of an antiparallel one, it is high. That follows from the fact that an unambiguous relation exists between the quantities M.sub.m and H.sub.m. In that connection, the following applies in the simplest case: EQU M.sub.m.multidot.H.sub.m =M.sub.m H.sub.m.
(In this case, the vector quantities are identified by bold script and the scalar quantities by non-bold scripts).
The magnetoresistance signal .DELTA.R of such a GMR multilayer system is then given by: EQU .DELTA.R=.DELTA.(1-cos .phi.).
It is evident from this equation that .DELTA.R adopts the same values for .phi.=.phi..sub.0 and .phi.=-.phi..sub.0. However, that means that the angle .phi. can be unambiguously determined only within a sector of 180.degree.. In addition, the angle sensitivity d.DELTA.R/d.theta.=.DELTA.sin.theta. is very low for .theta.=0 and .theta.=.pi., where .theta. is the angle between the direction of the external magnetic field H.sub.m and the reference direction stipulated by the magnetization of the bias layer (see the initially mentioned International Publication No. WO 94/17426, corresponding to U.S. Pat. No. 5,650,721).